Defect-driven nanostructuring of low-nuclearity Pt-Mo ensembles for continuous gas-phase formic acid dehydrogenation

Supported metal clusters comprising of well-tailored low-nuclearity heteroatoms have great potentials in catalysis owing to the maximized exposure of active sites and metal synergy. However, atomically precise design of these architectures is still challenging for the lack of practical approaches. Here, we report a defect-driven nanostructuring strategy through combining defect engineering of nitrogen-doped carbons and sequential metal depositions to prepare a series of Pt and Mo ensembles ranging from single atoms to sub-nanoclusters. When applied in continuous gas-phase decomposition of formic acid, the low-nuclearity ensembles with unique Pt3Mo1N3 configuration deliver high-purity hydrogen at full conversion with unexpected high activity of 0.62 molHCOOH molPt−1 s−1 and remarkable stability, significantly outperforming the previously reported catalysts. The remarkable performance is rationalized by a joint operando dual-beam Fourier transformed infrared spectroscopy and density functional theory modeling study, pointing to the Pt-Mo synergy in creating a new reaction path for consecutive HCOOH dissociations.

In the manuscript "Defect-driven nanostructuring of low-nuclearity Pt-Mo ensembles for continuous gas-phase formic acid dehydrogenation" Guo et al; report a defect-driven nanostructuring strategy through combining defect engineering of nitrogen-doped carbons and sequential metal depositions to prepare a series of Pt and Mo ensembles ranging from single atoms to sub-nanoclusters.They propose that low-nuclearity ensembles with unique Pt3Mo1N3 configurations deliver CO-free hydrogen at full conversion with unexpectedly high activity and remarkable stability in continuous gas phase formic acid decomposition.
In their short review, they discuss the merits and drawbacks of biomass-to-biochar catalyst conversion and the most relevant factors to consider in the synthesis stage for enhancing catalytic activities.
The manuscript is interesting, but the results found have been reported previously in the literature, so I do not find them to be of sufficient quality for the journal in which they are proposed for publication.For that main reason and the following points, I propose that it be rejected for publication in Nature Communications.
1.Although the introduction is well written, a real comparison with the most recent results obtained for the dehydrogenation of formic acid in liquid media is lacking.
2. The platinum particle distributions in Figure 2 are not faithful to reality.In the HAADF micrographs, platinum particles larger than 2-3 nm are observed (for example, in figure 2d), which are not contemplated in the particle distribution.
3. Assuming that the platinum atomic radius is 175 pm, how is it possible that particle distributions, with an average size smaller than the size of the platinum atomic radius, have been found (figure 2a and c). 4. Figure s4 shows the diffractogram of the obtained samples.A peak associated with the metallic platinum species is clearly visible, which means that the average crystal size is larger than 5 nm (the stopping limit of the technique). 5.However, the authors propose that the average size of the metallic particles, calculated by HR-TEM, figure 2e, is 0.66 nm.How do the authors explain this controversy?6. 4. The authors claim that the post-reaction sample shows no apparent modifications.However, there is an accentuation of the peak (110) of amorphous carbon, which could indicate a partial graphitisation of the carbonaceous support.On the one hand, the authors should show the XRD diffractogram from 10-90º 2 Theta, to observe the first carbon diffraction peak, and be able to compare them before and after the reaction, correlating these modifications when calculating the La and Lc parameters, and the packing factor, with the possible presence of carbonaceous leftover species.
For this reason, the work might not be published in Nature communications.
Reviewer #2 (Remarks to the Author): What are the noteworthy results?
The authors reported a defect-driven nanostructuring strategy through combining defect engineering of nitrogen-doped carbons and sequential metal depositions to prepare a series of Pt and Mo ensembles ranging from single atoms to sub-nanoclusters.When applied in continuous gas-phase decomposition of formic acid, the low-nuclearity ensembles with unique Pt3Mo1N3 configuration deliver CO-free hydrogen at full conversion with high activity and good stability.
Will the work be of significance to the field and related fields?How does it compare to the established literature?If the work is not original, please provide relevant references.
The reported work is interesting because it can produce CO-free hydrogen over 50 hours from formic acid.However, to have a significant impact on hydrogen applications using formic acid as the energy carrier, the authors need to consider the following: (1) Operating temperature: Even though the volumetric energy density of formic acid is higher than hydrogen gas, it is still very low compared to other liquid hydrogen carriers.The significant advantage of formic acid as a hydrogen carrier over other liquid hydrogen carriers (e.g., methanol or ethanol) is that it can reform at room temperature.As the operating temperature increases beyond the room temperature, the advantage of using formic acid as a hydrogen carrier diminishes.In order to have a significant impact, it is important that the catalyst needs to give both high activity and good stability at the ambient conditions (e.g., 25 oC and 1 atm) for the formic acid decomposition reaction.The reported Pt-Mo/NC catalyst is interesting, but it still requires a very high temperature of 100 oC to provide good performance.The real practical technological challenge for the formic acid decomposition reaction in hydrogen application is developing non-noble metal catalysts (or ultra-low precious metal catalysts) that can deliver CO-free hydrogen at ambient conditions over a long time.
(2) CO-free hydrogen: The gas stream from the reformer has to be free from CO gas (<10 ppm), or the catalytic performance of the fuel cells will be degraded significantly.Based on the given experimental data, it is hard to determine whether there were any ppm levels of CO gas or not.The author should provide information about CO gas concentration in ppm levels.Does the work support the conclusions and claims, or is additional evidence needed?
(1) The authors stated that "Mo atoms are first strongly coordinated by the N defects followed by the deposition of Pt, wherein controlled aggregation of Pt is realized by simply reducing the numbers of nitrogen defects."The authors need to clearly articulate how the N defects can coordinate with Mo atoms, and how this coordination chemistry can be affected by the concentration of N doping.Furthermore, there are no clear experimental data and analysis to explain how these N defectcontrolled Mo atoms interact with Pt atoms and control the final ensemble size of Pt.What is the exact role of Mo atoms in controlling the enzyme size of Pt in the overall synthesis process?
(2) Based on the performance data, Pt-Mo/NC0.07shows an improved activity compared to Pt/NC0.07.This improved activity could be originated from the ensemble size effect and/or the presence of Mo atoms.As the ensemble size changes and Mo atoms are introduced to Pt clusters, both their physicochemical and electronic properties will change and affect the catalytic performances.The authors provided DFT calculation data to explain the ensemble size effect and role of Mo atoms.However, their DFT data do not sufficiently address the size effect and the nature of the synergistic catalysis of the Pt-Mo ensembles.Could they prepare the samples with and without Mo while fixing the ensemble size?This comparison would allow them to isolate the Mo effect for example.
(3) The authors stated that "DB-FTIR study evidenced the different kinetic fingerprints of formic acid molecules with the distinct mono-and bimetallic sites as illustrated in Fig. S16."However, this reviewer cannot see how the DB-FTIR data shown in Figure 4 can lead to Figure S16.The DB-FTIR data do not reveal the distinct mono-and bimetallic sites.Instead, they simply show the different fingerprints of formic acid and its derivatives as the function of temperature.Based on the DB-FTIR data, it is very hard to make any conclusive statements about the degree of interaction between the specific sites of the catalyst and formic acid (and its derivatives).For example, the authors stated that "On the representative single-atom Pt sites, strong adsorption of formic acid was observed, but the deprotonation was much difficult."How can the authors prove that the deprotonation (of first H from O-H or second H from C-H?) was harder over the single-atom Pt sites than other catalysts?(4) The authors assigned the adsorption bands of the DB-FTIR data at 1718 and 1595 cm-1 as the C-O vibration of the molecularly adsorbed HCOOH and the O-C-O vibration due to the adsorbed formate species.If this is true, as the temperature increases the peak intensity of HCOOHad should decrease while the peak intensity of HCOOad should increase followed by the decrease.Do the DB-FTIR data show these trends?In general, the interpretation of the DB-FTIR data needs to be significantly improved and better match them to the predicted DFT-derived reaction mechanisms.
(5) Based on operando DB-FTIR data (Figure 4 c and d), it seems that both Mo/NC0.07 and Pt-Mo/NC0.07show very similar spectra.However, their activities are very different (Pt-Mo/NC0.07shows the best performance while Mo/NC0.07shows the worst performance).To explain this, the authors speculated that the abstraction of another H atom might be more energy-demanding than the dissociation of HCOOH.To support its speculation, the authors should show the energy profiles for the formic acid decomposition over the Mo clusters or Mo single atoms (whichever best represents the Mo/NC0.07sample).
(6) For the DFT energy profile of Pt3Mo1N3, the second H (from C-H bond) is deprotonated by chemisorbing to the nearest Pt site.However, in order for this H transfer to occur, this H must first move closer to the nearest Pt site.Because this H (from C-H) is located too far away from the nearest Pt site, this reviewer is not entirely convinced that such a transfer reaction can occur with the energy downhill.This should be a very unfavorable reaction because adsorbed HCOO needs to be stretched out to the nearest Pt site in an extreme degree.
Reviewer #3 (Remarks to the Author): The authors reported defect-driven nanostructuring strategy for the preparation of well-dispersed bimetallic Pt-Mo species on nitrogen-doped carbon, which have shown good catalytic activity in gasphase formic acid dehydrogenation.
Although the investigation comprises a systematic study of the material properties, catalytic evaluation, and mechanism, the following issues should be addressed before consideration for publication.
1. From the XPS spectra, the authors suggested the presence of Pt-N interactions.However, CH4 was used during the reduction process, which could lead to the formation of metal carbides (Catal.Sci.Technol., 2020, 10, 6790-6799).It is thus important to show whether Pt-C or Mo2C was formed.2. From the TEM studies, it was evident that some of the impregnated metal atoms were present as metal nanoparticles or metal clusters.However, surface sensitive techniques such as XPS did not show any Pt-Pt bond or Mo-Mo in the case of isolated impregnations (Pt/NC0.07 and Mo/NC0.07).These observations contradict the TEM studies, so the authors should discuss this anomaly.
3. Likewise, if the isolated impregnations do not show cluster formation, how can the bimetallic systems give rise to Pt-Pt clusters?4. The cluster sizes of the Pt-Mo/NC0.07(1-4 atoms) were estimated 10-30 times smaller than those of Pt-Mo/NC0.02(13-133 atoms), but why did the TEM images of both materials seem to show a high degree of agglomerated cluster? 5.The FTIR studies suggested that Mo/NC may be a major active species (or partially carbonized MoxCy species) since both the Mo/NC0.07 and Pt-Mo/NC0.07showed a similar dehydration pathway associated at elevated temperatures.This was contradicting with the plot "Fig.3a" which shows 100% CO2 selectivity for Mo/NC0.07,while its FTIR shows minor quantity of CO-related peaks at 2190 cm−1.
6.The authors stated that "gas-phase dehydrogenation is attractive considering the mild exothermicity and ease of catalyst separation as compared with the endothermic liquid-phase dehydrogenation (−15 vs. 29 kJ mol−1)"; however, vapor-phase dehydrogenation is operated at relatively higher temperatures.In the case of dilute feed (10% FA in water), it also generated unwanted steam, which is an endothermic process.Since 90% of water is being used in the feed, a significant amount of energy will be lost for vaporization.Have the authors proposed any heat recovery mechanism?If an inert gas such as helium is used as a carrier gas, are additional separation steps needed for hydrogen purification?
Minor comments: 7. The authors need to clarify whether metal loading is in weight percentage or atomic percentage.Text and table values were given in different units.
8. The references about the reviews for formic acid dehydrogenation should be updated.

Response to Reviewers
Comments in blue -Replies in black -Actions in bold, citationin italic

Reviewer #1
In the manuscript "Defect-driven nanostructuring of low-nuclearity Pt-Mo ensembles for continuous gas-phase formic acid dehydrogenation" Guo et al; report a defect-driven nanostructuring strategy through combining defect engineering of nitrogen-doped carbons and sequential metal depositions to prepare a series of Pt and Mo ensembles ranging from single atoms to sub-nanoclusters.They propose that low-nuclearity ensembles with unique Pt3Mo1N3 configurations deliver CO-free hydrogen at full conversion with unexpectedly high activity and remarkable stability in continuous gas phase formic acid decomposition.
In their short review, they discuss the merits and drawbacks of biomass-to-biochar catalyst conversion and the most relevant factors to consider in the synthesis stage for enhancing catalytic activities.
The manuscript is interesting, but the results found have been reported previously in the literature, so I do not find them to be of sufficient quality for the journal in which they are proposed for publication.For that main reason and the following points, I propose that it be rejected for publication in Nature Communications.
We sincerely thank the Reviewer 1 for his/her interest and his/her careful assessment of this work.However, we respectfully disagree with the Reviewer on that "the results found have been reported previously in the literature".The novel aspects of our work clearly distinguishing from the previous findings are briefly summarized as follows: i) Novel material design strategy.As stated in the Introduction of the previous manuscript, the rational design of low-nuclearity bimetallic catalysts is still a great challenge.The currently established approaches are mainly restricted to atomic layer deposition methods or confined pyrolysis of MOF/COF precursors containing hetero-atoms.Contrasting to the previous methods that generally lack control of the ensemble sizes, delicate tuning of the ensemble sizes in the subnano regime (from dual atoms to ca. 0.66 nm) is realized in our work.Our key idea is that, i) Mo atoms are introduced in the first deposition and strongly coordinated by the N defects after thermal treatment, ii) Pt atoms are sequentially introduced in the second deposition, iii) controlled aggregation of Pt is realized by reducing the numbers of nitrogen defects which act as the coordination sites for both Pt and Mo atoms.To our knowledge, it is the first time that defectdriven nanostructuring strategy in combination with sequential deposition of different metals is proposed for the design of bimetallic ensembles ranging from atomic dispersions to the subnano regime.Compared with the reported methods which require advanced facilities or preparation of complex precursors, our approach is more straightforward to practical applications.
ii) Outstanding catalytic performance of new Pt-Mo/NC catalytic system.It is the first time that bimetallic Pt-Mo catalysts were proposed for the dehydrogenation of HCOOH.In particular, the low-nuclearity Pt-Mo/NC0.07catalyst exhibit outstanding performance in the continuous gasphase HCOOH dehydrogenation as compared with the earlier reported analogues.Regarding the utilization efficiency of precious metals, Pt-Mo/NC0.07outperformed the other gas-phase dehydrogenation catalysts by ca.one order of magnitude in the total metal-based reaction rates, and reached close or even better values than those of the state-of-the-art liquid-phase dehydrogenation catalysts.(The Reviewer is kindly asked to refer to Fig. 3d and supplementary  Table 1).
iii) Firstly reported Pt-Mo synergy.We clearly demonstrated that the Pt-Mo ensembles in Pt-Mo/NC0.07superior catalytic performance in HCOOH dehydrogenation than any of the monometallic catalysts, including single Mo, Pt, and Pt clusters and nanoparticles.The superior catalytic performance of Pt-Mo/NC0.07 is further rationalized by combining operando DB-FTIR and DFT simulations, demonstrating that the unique Pt3Mo1N3 configuration can create a new reaction path with lower energy barrier for HCOOH dissociation by the cooperative catalysis of both Pt and Mo sites.
In fact, these innovative points have been recognized and praised by both Reviewers 2 and 3. We trust that the novelty of this work can be valuable and of high interest to the broad readers in material science, physical chemistry, and heterogeneous catalysis.To further address the Reviewer's questions/concerns, a detailed point-to-point reply has been provided in the following: 1.Although the introduction is well written, a real comparison with the most recent results obtained for the dehydrogenation of formic acid in liquid media is lacking.
Thank you for your praise and the comment.In the previous manuscript, we briefly compared the pros and cons of the liquid-and gas-phase processes for the dehydrogenation of formic acid, and the state-of-the-art liquid-phase dehydrogenation catalysts were also introduced.For instance, we have stated that 'Liquid-phase dehydrogenation is typically performed with the presence of homogeneous (e.g., Ru, Ir, and Fe-based organometallic complexes) [25][26][27] or Pd-based heterogeneous catalysts 28,29 .Both systems can afford high catalytic efficiency with the turnover frequency (TOF) reaching 0.31-5.97s −1 with typically negligible CO formation at low operation temperatures 30 .Nonetheless, the tedious recycling of the expensive organometallic complexes and the frequently reported deactivation of the heterogeneous catalysts due to the site blockage induced by trace CO prevent a prospective industrial process 31  Since the current work focused mainly on the gas-phase formic acid dehydrogenation, more attentions were paid on the introduction of gas-phase dehydrogenation catalysts which were drawn for the direct comparison on their catalytic performance.To address the Reviewer's comment, we have now accommodated the catalytic performances of the liquid-phase dehydrogenation catalysts from the most recent literatures for comparison with that of our developed catalyst in the revised piece (see supplementary Table 1).3. Assuming that the platinum atomic radius is 175 pm, how is it possible that particle distributions, with an average size smaller than the size of the platinum atomic radius, have been found (figure 2a and c).
Thank you for the comment.We should kindly remind the Reviewer that these images in Fig. 2 were acquired by HAADF-STEM technique.In principle, STEM imaging relies on the interactions of the atomic nucleus with the elastically scattered electrons.When sizes of the detecting objects decrease from large metal nanoparticles to single atoms, the white spots in the HAADF-STEM images are not the reflections of the metal atoms, but the atomic nucleus instead.In the case of Pt, the radius of Pt 2+ is much smaller than Pt atoms (80 vs. 138 pm).In addition, the outline of the white spots might also be influenced by the resolution of the recording camera.For these reasons, the sizes of atomically dispersed metals derived from the STEM is not exactly the same as those of the theoretical ones.Therefore, for single-atom catalysts, STEM imaging can provide valuable information based on the agglomeration of metal species, but cannot be used to determine the atomic size.
4. Figure s4 shows the diffractogram of the obtained samples.A peak associated with the metallic platinum species is clearly visible, which means that the average crystal size is larger than 5 nm (the stopping limit of the technique).
The Reviewer is correct that a small peak at 39.5 o 2 corresponding to the Pt(111) facet was found in the PXRD patterns of Pt-Mo/NC0.02.Accordingly, a small portion of metal nanoparticles up to about 4 nm in sizes was clearly observed in the HAADF-STEM images (Fig. 2e).Actually, this is within the detection limit of the PXRD technique, which is generally applicable to nanocrystalline powders with crystallite size above several nanometers (>3 nm, Material Characterization, 58 (2007) 883-891, ChemPlusChem, 88 (2023) e202300111; >2-2.5 nm, Catal.Lett., 145 (2015) 777).Even for the crystallites below 2-3 nm, diffraction patterns might be still visible, but probably displaying more broad diffraction peaks.For instance, broad diffraction peaks of CdS sphere particles of 1 and 2 nm were reported by Holder and Schaak (ACS Nano, 13 (2019) 7359-7365).O'Connell and Regalbuto also found the diffraction lines of Au particle as small as 1.2 nm (Catal.Lett., 145 (2015) 777-783).The Reviewer is kindly asked to the below figures cited from the above references.
'Fig. 2  5.However, the authors propose that the average size of the metallic particles, calculated by HR-TEM, figure 2e, is 0.66 nm.How do the authors explain this controversy?
The Reviewer is kindly reminded that the HAADF-STEM images showed the full size distributions of the metal species.The average particle size of 0.66 nm was the statistic number derived from the fitting line.In this specific catalyst, a small portion of larger nanoparticles up to about 4 nm were also observed which should be responsible for the diffraction of the Pt(111) facet.
6. 4. The authors claim that the post-reaction sample shows no apparent modifications.However, there is an accentuation of the peak (110) of amorphous carbon, which could indicate a partial graphitisation of the carbonaceous support.On the one hand, the authors should show the XRD diffractogram from 10-90º 2 Theta, to observe the first carbon diffraction peak, and be able to compare them before and after the reaction, correlating these modifications when calculating the La and Lc parameters, and the packing factor, with the possible presence of carbonaceous leftover species.
Thank you for your valuable comments.Since the metallic species are generally regarded as the active sites in the decomposition of HCOOH, we mainly focused on these in the previous manuscript.Following your suggestion, we have replotted the PXRD patterns showing the range at 10-80 o 2 ( supplementary Fig. 16a).Besides, we have estimated the La and Lc values for the carbon supports before and after the catalytic tests, and these parameters are not significantly different.To further exclude the presence of carbonaceous deposits, we have performed additional Raman spectra analyses, which showed negligible changes between the fresh and used catalysts (supplementary Fig. 16b).
Accordingly, we have been rephrased the sentences as follows: Page 15, lines 18-22: 'Pt-Mo/NC0.07 after the stability test was thoroughly characterized by different techniques.PXRD and Raman spectra analyseis revealed the same amorphous nature of the spent catalyst and no significant alternation of the carbon carrier (Supplementary Fig. 16).Nno diffractions of Pt-and/or Mo-related compounds were detected by PXRD (Fig. S11), suggesting that these metal species remained highly dispersed.' The Scherrer formula is used to obtain the crystallite height (Lc) and the crystallite width (La) : Lc = ( x) / (002 x cos002), La = ( x) / (100 x cos100), wherein  is the Scherrer constant (assuming 0.89),  is the wavelength (0.154056 nm),  is the full width at half maximum, and  is the Bragg angle.
These values were calculated to be Lc = 0.93 nm, La = 1.59 nm for the fresh catalyst, and Lc = 0.87 nm, La = 1.51 nm for the use catalyst.

Reviewer #2
What are the noteworthy results?
The authors reported a defect-driven nanostructuring strategy through combining defect engineering of nitrogen-doped carbons and sequential metal depositions to prepare a series of Pt and Mo ensembles ranging from single atoms to sub-nanoclusters.When applied in continuous gas-phase decomposition of formic acid, the low-nuclearity ensembles with unique Pt3Mo1N3 configuration deliver CO-free hydrogen at full conversion with high activity and good stability.
Will the work be of significance to the field and related fields?How does it compare to the established literature?If the work is not original, please provide relevant references.The reported work is interesting because it can produce CO-free hydrogen over 50 hours from formic acid.However, to have a significant impact on hydrogen applications using formic acid as the energy carrier, the authors need to consider the following: The Reviewer 2 is warmly thanked for the very careful assessment of our work and for recognizing the merits of this piece.We also thank you for your critical comments that are extremely valuable for us to further improve the quality of our manuscript.In this revision, we have conducted additional experiments and DFT calculations to address your concerns and a detailed point-to-point reply has been provided as follows: (1) Operating temperature: Even though the volumetric energy density of formic acid is higher than hydrogen gas, it is still very low compared to other liquid hydrogen carriers.The significant advantage of formic acid as a hydrogen carrier over other liquid hydrogen carriers (e.g., methanol or ethanol) is that it can reform at room temperature.As the operating temperature increases beyond the room temperature, the advantage of using formic acid as a hydrogen carrier diminishes.In order to have a significant impact, it is important that the catalyst needs to give both high activity and good stability at the ambient conditions (e.g., 25 oC and 1 atm) for the formic acid decomposition reaction.The reported Pt-Mo/NC catalyst is interesting, but it still requires a very high temperature of 100 oC to provide good performance.The real practical technological challenge for the formic acid decomposition reaction in hydrogen application is developing non-noble metal catalysts (or ultra-low precious metal catalysts) that can deliver COfree hydrogen at ambient conditions over a long time.
We fully agree with the Reviewer's opinions.Indeed, the activity of the best-developed catalyst Pt-Mo/NC0.07markedly deteriorated below 100 o C, and it is still far from the viewpoint of practical applications.Nonetheless, it still represents one of the best solid catalysts for the gas-phase dehydrogenation of HCOOH reported to date.Regarding the utilization efficiency of precious metals, Pt-Mo/NC0.07outperformed the other gas-phase dehydrogenation catalysts by ca.one order of magnitude in the total metal-based reaction rates, and reached close or even better values than those of the state-of-the-art liquid-phase dehydrogenation catalysts.(The Reviewer is kindly asked to refer to Fig. 3d and Supplementary Table 1).
As pointed by the Reviewer, the development of non-noble metal catalysts is a more practical direction.In this context, we trust that the principle of the proposed strategy of defect-driven nanostructruing of bimetallic low-nuclearity ensembles might stimulate a pulse for the delicate design of advanced catalytic materials comprising multi-atoms and bring new opportunities to tackle this challenge.These messages have been accommodated in the revised Conclusion section.

Page 25, lines 15-19: 'To develop a practical HCOOH-to-H2 technology, it is still imperative to design more efficient catalytic materials based on non-noble metals which can operate ideally at ambience conditions. In this scenario, the developed defect-driven nanostructuring approach may offer new opportunity to tackle this challenge through rational design of more sophisticated lownuclearity heteroatom ensembles.'
(2) CO-free hydrogen: The gas stream from the reformer has to be free from CO gas (<10 ppm), or the catalytic performance of the fuel cells will be degraded significantly.Based on the given experimental data, it is hard to determine whether there were any ppm levels of CO gas or not.The author should provide information about CO gas concentration in ppm levels.
Thank you for your valuable comments.In our previous analysis method, N2 was used as the carrier gas for the gas chromatograph (GC).As can be seen from supplementary Fig. 15a, the trace CO can hardly be detected.However, the zoom figure in supplementary Fig. 15a' showed a small bump around 5.5 min, which should be contributed by CO.Nonetheless, this very small peak was intervened by the tailing of N2 signal and thus the area could not be well integrated.To overcome this drawback, we have replace N2 with He as the carrier gas for the GC analysis.Also, the full spectra show no obvious CO signals (Supplementary Fig. 15b), while tiny CO peaks can indeed be found in the zoomed figure (Supplementary Fig. 15b').By referring to the standard gases, we have now quantified the trace CO during stability test on Pt-Mo/NC0.07.The average CO content was determined to be 15.5 ppm.Accordingly, we added the quantification method of trace CO in the Experimental section, and rephased our claim by replacing the term 'CO-free hydrogen' with 'high-purity hydrogen'.
Page 3, lines 7-11: When applied in continuous gas-phase decomposition of formic acid, the lownuclearity ensembles with unique Pt3Mo1N3 configuration deliver high-purityCO-free hydrogen at full conversion with unexpected high activity of 0.62 molHCOOH molPt −1 s −1 and remarkable stability, significantly outperforming the previously reported catalysts.
Page 15, lines 15-18: 'The activity gradually increased after a few hours of stabilization and slightly fluctuated at ca. 90-96%, while trace CO of 15.5 ppm in averageCO2 was detectedthe only detectable product in our gas chromatography (Supplementary Fig. S15 12).Specifically, several diffraction facets corresponding to Mo2C were observed on Mo/NC0.004, and the contribution of surface Mo 2+ /Mo 4+ species were also confirmed for this sample.Altogether, the above findings suggest that the N defect plays a critical role in stabilizing the Mo species against sintering.
Furthermore, there are no clear experimental data and analysis to explain how these N defectcontrolled Mo atoms interact with Pt atoms and control the final ensemble size of Pt.What is the exact role of Mo atoms in controlling the enzyme size of Pt in the overall synthesis process?
Thank you for your good question.We think that the key role of N-coordinated Mo species in influencing the ensemble sizes of Pt species is through modulating the number of N defects available for coordination with Pt atoms.This is supported by the following two experimental observations: (1) More severe aggregation of Pt species is observed for the bimetallic Pt-Mo/NC0.07catalysts as compared with Pt/NC0.07, (2) and the sizes of Pt-Mo ensembles grow at decreasing numbers of the N defects in the order of Pt-Mo/NC0.13< Pt-Mo/NC0.07< Pt-Mo/NC0.02.
Since our characterizations suggest the atomic metal dispersion on both Mo/NC0.07 and Pt/NC0.07,and the clustering is only observed for Pt-Mo/NC0.07,we trust it is reasonable to attribute the growing Pt sizes of Pt-Mo/NC0.07 to the reduced number of the free N defects for coordination.
Following this reasoning, it can also well explain the increasing Pt sizes at the decreasing N defects for the bimetallic catalyst series.As we adopted the sequential deposition method to prepare the bimetallic catalysts (first Mo and then Pt), once the N defects are coordinated with the Mo atoms, there are no sufficient N sites for the coordination of Pt atoms that are consequently more prone to aggregation.
On the other hand, we also agree with the Reviewer that the N-coordinated Mo atoms might interact with the Pt species and potentially influence the clustering behavior.We have tentatively compared the formation energy of Pt4N3 and Pt3Mo1N3 with similar tetrahedron configurations (Supplementary Fig. 14).The results show that Pt3Mo1N3 possesses a slightly higher formation energy than Pt4N3 (−16.84 vs. −16.34eV), suggesting that the presence of Mo is beneficial for the formation of low-nuclearity Pt-Mo ensembles.
Supplementary Fig. 14.The configurations of Pt4N3 and Pt3Mo1N3, accompanied with the formation energy.
Taking into all the above discussions, we stick to our previous hypothesis on the key role of N-coordinated Mo atoms, but also stressed the potential role of Pt-Mo interactions in affecting the clustering behavior for which more evidences are needed.
We have accommodated the new calculated results in the revised piece.
Page 14, lines 7-11: 'In addition, we have tentatively compared the Ef of Pt4N3 and Pt3Mo1N3 with similar tetrahedron configurations (Supplementary Fig. 14).The results showed that Pt3Mo1N3 possesses a slightly higher Ef than Pt4N3 (−16.84 vs. −16.34eV), suggesting that the presence of Mo is thermodynamically beneficial for the formation of low-nuclearity Pt-Mo ensembles.' (2) Based on the performance data, Pt-Mo/NC0.07shows an improved activity compared to Pt/NC0.07.This improved activity could be originated from the ensemble size effect and/or the presence of Mo atoms.As the ensemble size changes and Mo atoms are introduced to Pt clusters, both their physicochemical and electronic properties will change and affect the catalytic performances.The authors provided DFT calculation data to explain the ensemble size effect and role of Mo atoms.However, their DFT data do not sufficiently address the size effect and the nature of the synergistic catalysis of the Pt-Mo ensembles.Could they prepare the samples with and without Mo while fixing the ensemble size?This comparison would allow them to isolate the Mo effect for example.
Thank you for your comments and valuable suggestions.To address these questions, two actions have been taken in this revision: First, we have attempted to prepare Pt/NC catalysts with the average Pt particle sizes close to those of Pt-Mo/NC0.07,following the same recipe.Since Pt/NC0.07 in the absence of Mo showed predominantly single Pt atoms even after reduction at a high temperature of 973 K, we have adopted NC0.02 as the carrier because we envision that the lower number of N defects may facilitate the agglomeration of Pt species.By carefully tuning the reduction temperature of 573 K, We obtained a sample -Pt/NC0.02 with similar particle size distributions as those of Pt-Mo/NC0.07,with an average size of 0.31±0.27nm (Supplementary Fig. 26).
Supplementary Fig. 26.HAADF-STEM images of Pt/NC0.02 and the particle size distributions.This reference catalyst was prepared following the same recipe as Pt/NC0.07but with a lower interaction between the specific sites of the catalyst and formic acid (and its derivatives).For example, the authors stated that "On the representative single-atom Pt sites, strong adsorption of formic acid was observed, but the deprotonation was much difficult."How can the authors prove that the deprotonation (of first H from O-H or second H from C-H?) was harder over the singleatom Pt sites than other catalysts?
Thank you for the question.The intensity of the absorption bands in DB-FTIR spectra can reflect the strength of the interactions of the surface adsorbed species with the catalyst surface.
According to the previous literatures, the absorption peaks at 1718 and 1595 cm - In our DB-FTIR experiments (Fig. R2), the catalysts were first saturated with HCOOH vapor at room temperature before collecting the spectra.Once saturated, both physically and chemically adsorbed HCOOH species in significant amounts were readily detected on Pt-Mo/NC0.07.The gaseous or physically adsorbed HCOOHad species can be gradually removed by purging with flowing N2, while the chemically adsorbed HCOOad species did not markedly change in intensity (A slightly enhanced intensity of the peak at 1595 cm -1 was indeed found at the beginning during N2 purging).We think the absence of obvious increasing intensity of the band at 1595 cm -1 might be due to relatively fast kinetics for the H dissociation.To slow down the kinetics, collecting the spectra at liquid N2 temperature is an option.Nonetheless, the high freezing point of HCOOH (281 K) makes it unsuitable for such experiments.
To further substantiate the DB-FTIR results, we have performed additional DFT simulations on the reaction mechanism of HCOOH dehydrogenation on single-site MoN3 catalyst (Mo coordinated with triple pyridinic-N defects).In general, the DB-FTIR results fit well with the DFT calculations.The Reviewer is kindly asked to refer to the replies to Question 5. shows the best performance while Mo/NC0.07shows the worst performance).To explain this, the authors speculated that the abstraction of another H atom might be more energy-demanding than the dissociation of HCOOH.To support its speculation, the authors should show the energy profiles for the formic acid decomposition over the Mo clusters or Mo single atoms (whichever best represents the Mo/NC0.07sample).
Thank you for your comments.To support our claim, we have simulated the reaction coordinate of HCOOH decomposition on MoN3 sites (Mo coordinated with triple pyridinic-N defects) by DFT (Supplementary Fig. 24).We found that the activation of the first H atom in HCOOH is relatively easier with a low energy barrier of 0.19 eV, while the energy barriers for the deprotonation of the second H atom in HCOO* and the desorption of COO* are 1.59 and 1.35 eV, respectively.These findings thus fully support our speculations from DB-FTIR, that " The mono-metallic Mo catalyst also showed a higher propensity toward HCOOH adsorption and its dissociation but displayed the poorest decomposition activity.This suggested that other fundamental steps, such as the abstraction of another H atom, might be more energy-demanding than the dissociation of HCOOH (vide infra).Another explanation could be the too strong adsorption of HCOOH on the Mo sites as indicated by the much higher desorption temperatures in the operando DB-FTIR study."(Page 20, Lines 9-14) Furthermore, comparison of the energy profiles shows that both the activation of the H atom in the adsorbed HCOO* species and the desorption of the surface COO* species were more energy-demanding on MoN3 than on Pt3Mo1N3 (1.59 vs. 1.18 eV, and 1.35 vs. 1.17 eV).These findings also support our claim, that "In contrast, these two fundamental steps of formic acid adsorption and dissociation were both more favorable on the low-nuclearity Pt-Mo ensembles, thus accounting for the highest activity" (Page 20, Lines 14-16).We have strengthened the above claims by accommodating the new DFT calculation results in the revision.

Reviewer #3
The authors reported defect-driven nanostructuring strategy for the preparation of well-dispersed bimetallic Pt-Mo species on nitrogen-doped carbon, which have shown good catalytic activity in gas-phase formic acid dehydrogenation.
Although the investigation comprises a systematic study of the material properties, catalytic evaluation, and mechanism, the following issues should be addressed before consideration for publication.
We highly appreciate the Reviewer 3 for the careful evaluation of this work, and thank you for your support of publication of this piece after addressing the following issues.The detailed replies to your questions have been provided point-by-point as follows: 1. From the XPS spectra, the authors suggested the presence of Pt-N interactions.However, CH4 was used during the reduction process, which could lead to the formation of metal carbides (Catal.Sci.Technol., 2020, 10, 6790-6799).It is thus important to show whether Pt-C or Mo2C was formed.12b).On the contrary, the Mo 3d XPS spectra of all our Mo-containing samples showed the exclusive Mo 6+ species at a much higher binding energy of 232.4-232.8eV.Therefore, we trust that the Mo atoms were mainly coordinated with the N defects.To further understand the potential role of N defects in stabilizing the Mo atoms, we have intentionally tune the numbers of N defect in NCx carriers, which were then used to prepared additional Mo/NCx catalysts following the same recipe of Mo/NC0.07.Our thorough characterization results combining HAADF-STEM, PXRD, AND XPS showed that Mo2C can be formed only for carrier with the least number of N defect, i.e., NC0.004 (Supplementary Figs.11,12).These observations highlight the importance of sufficient N defective sites in stabilizing the Mo atoms and against sintering during the high-temperature annealing treatment.
In addition, we have performed additional DFT calculations to understand how the numbers of N defects can influence the stabilization of single Mo atoms (Supplementary Fig. 10).As our characterization showed the more prominent role of pyridinic N in anchoring the metal atoms, multiple pyridinic N defective sites were considered that may also reflect the increasing concentrations of N doping.As shown in Supplementary Fig. 10a, the formation energy increased with the increasing numbers of N atoms in the MoNx entities, reflecting the increasing stability.Bader charge analysis reveals the electron-deficient states of the Mo atoms in all these configurations, agreeing well with the ionic nature of the Mo species based on Mo 3d XPS and XANES observations.
The above new results have been accommodated into the revised manuscript.
Page 13, lines 6-21: 'To understand how the N defects can coordinate with the Mo atoms, the formation energies (Ef) as well as Bader charges of Mo species stabilized by multiple pyridinic-N defects were calculated by DFT (Supplementary Fig. 10).A higher Ef was observed with the increasing numbers of the N anchor in the MoNx entities, suggesting the higher stability.Bader Supplementary Fig. 6.Additional HAADF-STEM images of the representative mono-and bimetallic catalysts.
5. The FTIR studies suggested that Mo/NC may be a major active species (or partially carbonized MoxCy species) since both the Mo/NC0.07 and showed a similar dehydration pathway associated at elevated temperatures.This was contradicting with the plot "Fig.3a" which shows 100% CO2 selectivity for Mo/NC0.07,while its FTIR shows minor quantity of CO-related peaks at 2190 cm−1.
Thank you for your very delicate observation and your comments.Indeed, very subtle peaks at 2190 cm -1 can be observed on Mo/NC0.07 at increasing temperatures.The intensity of these peaks was much weaker as compared with that observed on Pt-Mo/NC0.07.The Reviewer is reminded that the CO selectivity was less than 0.5% at 473 K on Pt-Mo/NC0.07.Given the much lower activity of Mo/NC0.07 as well as the extremely weak peak intensities at 2190 cm -1 , it is reasonable that trace CO cannot be detected by our GC (detection limit of 10 ppm).On the other hand, it might be also possible that the desorption of this specie adsorbed at 2190 cm -1 is much difficult on Mo/NC0.07than on Pt-Mo/NC0.07.In this revision, we have simulated the reaction coordinate of HCOOH dehydrogenation on MoN3 sites (Mo coordinated with triple pyridinic-N defects) by DFT (Supplementary Fig. 24).Comparison of the energy profiles shows that both the activation of the H atom in the adsorbed *HCOO species and the desorption of the surface *COO species are more energy-demanding on MoN3 than on Pt3Mo1N3 (1.59 vs. 1.18 eV, and 1.35 vs. 1.17 eV).This might also partly explain why no CO was formed on Mo/NC0.07.
Supplementary Fig. 24.a, The relative energy of HCOOH decomposition on the MoN3 singleatom model, and b, the side view of DFT-optimized adsorption configurations of the intermediates.The reaction simulation on Pt3Mo1N3 was provided in a for reference.
These new results have been added and discussed in the revision to support our claim.
Page 24, Lines 2-8: 'To further support the synergistic effect, DFT simulations on the other monometallic sites, i.e., MoN3 (single Mo atom coordinated with triple pyridinic-N sites) and Pt4N3 (tetrahedron Pt clusters stabilized in triple pyridinic-N sites) were performed.Comparison on the reaction coordinates revealed higher energy barriers for i) the activation of *HCOO and desorption of *COO on MoN3 (Supplementary Fig. 24), and ii) the deprotonation of HCOOH on Pt4N3 (Supplementary Fig. 25), as compared with the respective elemental steps on the Pt3Mo1N3 sites.' 6.The authors stated that "gas-phase dehydrogenation is attractive considering the mild exothermicity and ease of catalyst separation as compared with the endothermic liquid-phase dehydrogenation (−15 vs. 29 kJ mol−1)"; however, vapor-phase dehydrogenation is operated at relatively higher temperatures.In the case of dilute feed (10% FA in water), it also generated unwanted steam, which is an endothermic process.Since 90% of water is being used in the feed, a significant amount of energy will be lost for vaporization.Have the authors proposed any heat recovery mechanism?If an inert gas such as helium is used as a carrier gas, are additional separation steps needed for hydrogen purification?
Thank you for your questions.Although diluted aqueous solutions of HCOOH, sometime in conjunction with additives such as sodium formate, are frequently used in the liquid-phase decomposition, we fully agree with the Reviewer 2 on that it is an ideal approach to use pure HCOOH for the gas-phase dehydrogenation.As pointed out by the Reviewer, this is a more energy-saving way to avoid additional energy input for water heating or separation.For our experiments, HCOOH/H2O solution was adopted simply because we were not able to deliver an extremely low feed of pure HCOOH in a stable manner with the syringe pump available in our lab.
As an alternative, a diluted HCOOH solution was adopted instead in order to ensure a more reliable feeding rate of HCOOH.
Minor comments: 7. The authors need to clarify whether metal loading is in weight percentage or atomic percentage.Text and table values were given in different units.
The metal loadings reported were in weight percentage based on ICP analysis.We are sorry for the inconsistency.The unit of the metals in Supplementary Table 2 has been changed from at.% to wt.% in the revision.

Fig. 2 .
Fig. 2. Characterization of the key catalysts.a-e, HAADF-STEM images with the particle size distributions.f,g, Normalized XANES spectra with the Pt and Mo foils references.h,i, EXAFS and the fitting results.

Fig. 2 .
Fig. 2. Characterization of the key catalysts.a-e, HAADF-STEM images with the particle size distributions.f,g, Normalized XANES spectra with the Pt and Mo foils references.h,i, EXAFS and the fitting results.
).' Page 31, lines 7-8: 'Trace CO was quantified by referring the standard gases (10 and 50 ppm CO/He).' formation energy of the MoNx entities increased with the increasing numbers of N anchors, reflecting the increasing stability.Bader charge analysis reveals the electron-deficient states of the Mo atoms in all these configurations (Supplementary Fig.10b), agreeing well with the ionic nature of Mo based on the Mo 3d XPS and XANES observations.Supplementary Fig.10.a, The formation energy and Bader charge of different MoNx (x = 2-4) entities, and, b, the configurations and charge density plots of MoNx.To further address the impact of the numbers of N defect on the coordination chemistry of the Mo species, we have prepared additional two Mo/NCx catalysts (5 wt.% Mo, x = 0.004 and 0.02) as references following the same recipe for Mo/NC0.07.HAADF-STEM images clearly show that a few particles were formed on Mo/NC0.02 and more severe aggregation occurs for Mo/NC0.004(Supplementary Fig. 11).The lattice fringes corresponding to the (101) facets of Mo2C is verified for Mo/NC0.004.These observations were further corroborated by PXRD and Mo 3d XPS analyses (Supplementary Fig.

Fig. S17 .
Fig. S17.The a, adsorption and b, dissociation energies of HCOOH at different Pt model systems.The corresponding side-view configurations were shown in the insets.

Fig. 5 .
Fig. 5. Mechanistic insights from DFT. a, Energy profiles for the decomposition of formic acid on the different model catalysts.b, Side view of DFT-optimized key adsorption configurations.

Table S1 .
Comparison on the performance between Pt-Mo/NC0.07and the literature reported catalysts in the gas-phase dehydrogenation of formic acid.H2 selectivity.b rate in mol of HCOOH converted per mol of precious metals per second.c Deactivation rate defined by the change in HCOOH conversion in percentage per hour during stability tests.*This work.

Table 1 .
Comparison on the performance between Pt-Mo/NC0.07and the literature reported solid catalysts in the gas-and liquid-phase dehydrogenation of formic acid.
a H2 selectivity.b rate in mol of HCOOH converted per mol of precious metals per second.c Deactivation rate defined by the change in HCOOH conversion in percentage per hour during stability tests.*This work.FA: formic acid, SF: sodium formate.n.d.: not detected.
Thank you for the relevant comment.As pointed out by the Reviewer, Mo2C is a potential product when Mo-based materials are carbonized in CH4-containing stream at high temperatures.According to the literatures (Inorg.Chem., 62 (2023) 653-658, ACS Sustain Chem.Eng., 7 (2019) 18375-18383, Dalton Trans., 51 (2022) 17547-17552), the presence of Mo2C species can be detected by using XPS.Typical Mo 2+ species at the binding energy of ca.228.3-228.7 eV, corresponding to Mo 3d orbital, have been reported.This has also been confirmed from the XPS result of a commercial Mo2C sample (Supplementary Fig.